Members of the transcription factor NF-?B and inhibitor I?B families are the key regulators of diverse array of cell signaling pathways that have profound impact on cell physiology, in particular, immuno-modulation, survival and proliferation of cell. NF-?B1 (also called p105/p50) and NF-?B2 (also called p100/p52) belong to both the NF-?B and I?B families. p105 and p100 are the precursors which processed into smaller products, p50 and p52, members of the NF-?B family. Precursors function as inhibitors of NF-?B, including their own processed products. Processing must be tightly regulated to generate appropriate ratio of unprocessed and processed forms. Many human diseases, such as autoimmune diseases and cancer, are linked to unregulated processing of p100 and p105. p105 and p100 divulge highly complex regulatory events in the NF-?B signaling module by generating all of NF-?B p50 and p52 proteins and inhibiting nearly half of all five NF-?B proteins. Like the prototypical I?B inhibitors, I?B?,-? and -?, p105 and p100 inhibitors also undergo complete degradation releasing NF-?B transcription factors. However, complete degradation events are difficult to observe in the backdrop of the processing events and thus are less appreciated. Processing of p105 is constitutive whereas that of p100 is inducible. This proposal aims to study the biochemical mechanisms of processing and complete degradation and coupling between these two events. Our hypothesis, derived from our preliminary results and published reports, is that the fundamental regulation of processing or lack of it is guided by several competing inter- and intra-domain interactions in p105 and p100, and these interactions can be altered by signaling. We propose that p105-self interaction induces a structural state that is conducive to constitutive processing whereas p100 attains that state only through signaling. We further propose that as the fully assembled NF-?B inhibitory complexes, the processing sites of p105 and p100 are masked. p105 and p100 in these assembled complexes must undergo complete degradation by cell signaling to liberate bound NF-?B. We will test our hypothesis by i) determining the three-dimensional structures of p105/p100 bound to NF-?B using x- ray crystallography, ii) identifying interaction strategies and energies of the p105/p100:NF-?B complexes, and finally, iii) studying the processing and degradation of the precursors.